We’ve see a lot of clever irrigation devices for gardeners being made with the Pi, but PiPlanter is the most complete (and the best documented) system we’ve seen so far. It does far more than simple irrigation. PiPlanter monitors temperature, ambient light, ambient humidity and soil humidity; it outputs that data to a MySQL database, controls a pump to water the plants depending on that data, and outputs the data as graphs and text. (It also tweets that text and uploads the graphs to Flickr hourly so that Devon, the PiPlanter’s owner, can keep an eye on things.)

Measuring soil moisture with two nails and in DC resistive divider isn’t going to work for long. Electrolysis will soon mess with it. The ionic concentrations of the soil will change if you add fertiliser, which will mess with it too.

We used to use LM1830 to give wet/dry indication, it puts AC across the probe so reducing the electrolysis. We used 6.5mm stereo headphone plugs wrapped in gauze as a sensor for a system designed to keep cuttings leaves damp. This was a huge improvement on the square of fine mesh on a beam balance with a micro switch. Frogs at least don’t sit on it, or the gaped carbon rod sensor that spiders wove webs across.

For the last 10 years we have only measured the air temperature and based the misting rate on that. For plants outside we have a rain sensor and we water them if it not raining.

It looks like he has had a bundle of fun setting that all up and that’s great, but I can testify to the large number of pitfalls to making such a system that works reliably for years and to which you would trust a commercial crop.